Motor controllers are used to control very high power alternating current (AC) induction motors. Typically, motor controllers for these high power motors include large transistors, such as insulated gate bipolar transistors (IGBT), capable of operating at high current levels often exceeding 1,000 amps. The transistors are used to synthesize AC waveforms to drive the motor at a desired speed, torque and power consumption. The transistors receive a low level control signal, which rapidly switches the transistors on and off to produce the desired motor current.
It is often necessary to analyze the current waveforms of the transistor. However, typical high power motor controllers have the transistors connected to large bus bars within a rigid housing. Thus, it is difficult to access the transistor terminals for testing without disassembling the motor controller. Moreover, current typically must be measured while the transistor is connected to the controller circuitry and operating. Thus, even if the controller is disassembled, the transistors must be suitably connected to the controller using large conductor extensions that are capable of carrying high current levels. Large conductor extensions would be burdensome and may affect the operation of the controller and thus the accuracy of the current measurement.
There are a number of methods and devices for measuring high current waveforms. One such known device is a current transformer. Typically, current transformers have an inductive coil that surrounds a large conductor extension attached to the transistor. The current transformer steps down the high current to a fractional current value at a smaller conductor attached to the current transformer. The current at this smaller conductor can be measured by a standard meter or oscilloscope. Current transformers can handle large current levels, however, they cannot be used to test direct current DC). Moreover, current transformers are physically large and difficult to connect to the controller. Further, typical current transformers may not be suitable for testing transistor operation because of they introduce additional inductance into the current path.
Another known device is a coaxial shunt providing a high wattage resistor divider in a metallic finned cylinder. This device has a good frequency response and is capable of handling high currents. However, like current transformers, these devices are also difficult to connect to the controller circuitry because of their physical size and significant inductance.
Still another known device for measuring high current is a Hall effect current sensor. Hall effect sensors are solid-state devices that are very sensitive to the presence of magnetic fields. As known in the art, the magnetic field detected by the sensor can be used to analyze current. These devices are not well-suited from testing high current industrial controllers because they have a limited range and bandwidth and are difficult to position in the current path.
Accordingly, it would be desirable to have a device for use in analyzing current waveforms of high power devices that can be easily connected to the devices without significant disassembly or adding significant inductance to the circuit.
The present invention provides a laminated shunt with a low profile such that it can be easily disposed within small gaps between loosened assembled components. Moreover, its laminated construction and thin, wide geometry allows it to be used with low inductance AC circuits.
Specifically, the low profile laminated shunt of the present invention has an electrically insulated substrate defining a flat, planar body having terminal and measurement ends. At the terminal end, first and second conductive terminals are fixed to opposed surfaces of the substrate and have openings adjacent to and concentric with an opening in substrate. At the measurement end, two leads attach a sensing element to the substrate. First and second lead layers connect each lead of the sensing element to one of the first and second terminals. A shunt layer disposed between and substantially in the plane of the first and second lead layers connects the first and second terminals together. The shunt layer has a finite electrical resistance lower than that of the sensing element.
Insulating layers are disposed on each side of the shunt layer. The insulating layers of the shunt can include an adhesive at one or both surfaces for joining the insulating layers to the substrate. Moreover, the terminals, lead layers and shunt layer can be a metallic foil having an adhesive backing. In this case, the adhesive of the insulating, lead and shunt layers retain the layers in close proximity to each other and to the substrate, giving the shunt a laminated construction.
The present invention provides a low profile shunt that can be used to analyze electrical devices, such as motor controllers, without significant, if any, disassembly. The wide construction and low profile allow the shunt to be inserted between small gaps at electrical terminals of assembled components that are separated by loosening their connection or flexing one or more of the components. Moreover, the wide, low profile geometry gives the shunt desirable current handling capabilities without introducing a significant amount of inductance.
The present invention provides another object and advantage of a shunt that can be used to accurately analyze current waveforms using standard meters and oscilloscopes. Since the shunt layer has significantly less resistance than the sensing element, only a relatively small amount of current (preferably {fraction (1/1,000)}th of the total) passes through the sensing element. Thus, even if a device operates at a high current, the shunt of the present invention reduces the current to be analyzed to a level suitable for ordinary meters.
The shunt of the present invention provides an additional object and advantage of providing a shunt that can be used to analyze current waveforms of low inductance circuits.
Still further, the shunt can be used to measure current in both AC and DC circuits.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.